INTRODUCTION 7 



ordinary acceptation, we understand the appear- 

 ance of visible complexity. But when we speak of 

 the visibility of the resulting complexity, we use a 

 subjective term, the value of which is relative to 

 the human eye. Going further into the matter, we 

 must break up the conception into two parts, and 

 distinguish between the actual production of com- 

 plexity and the mere transformation of complexity 

 from a condition invisible to us into complexity 

 visible to our senses. 



' The two kinds of development I have indicated 

 bear a relation to each other that recalls the old 

 opposing doctrines of preformation and epigenesis, 

 the alternatives of a time when it was a task 

 perhaps the only possible task to record the com- 

 pleted results of the stages in development as they 

 became complete in fact, to record the externally 

 visible changes of shape. In this descriptive in- 

 vestigation of the development of external form, 

 epigenesis, the successive formation of new shapes, 

 gained a complete victory over evolution, the mere 

 becoming visible of pre-existing details of shape. 



' The closer investigation of embryonic develop- 

 ment that is necessary in a search for causes 

 brings us once more against the old alternatives, 

 and compels us to a closer scrutiny of them. 



' In this, if we still retain the old terms, epigenesis 

 would mean not merely the building up of com- 

 plicated form through the agency of a substratum, 

 apparently simple, but perhaps with an extra- 

 ordinarily complicated, minute structure, but, in the 

 strictest sense of the term, the new formation of 



